It reminds you to read your own posts in the context of the thread...
It's not perfect, but if you "Return to discussion" after posting, that does count as reading the post and then it will be white. I would classify it as a minor annoyance.
All visited threads with new posts are red. All visited threads with no new posts are white. If I make a new post, the thread is red. I think it should be white, because I have already read the message that I posted.
Is it just me or is this a bug?